The invention relates to a friction ring for a synchronizing unit of a selectable variable ratio gear transmission, to a synchronizer ring, to a synchronizing unit as well as to a variable ratio gear transmission for a vehicle in accordance with the preamble of the independent claims.
In a mechanical, selectable variable ratio gear transmission, e.g. in vehicle transmissions, synchronizer rings serve to synchronize the relative speeds which occur between the gear wheel and the gear shaft on a gear change to one another. The synchronization is achieved in this respect by friction between the corresponding friction partners. The function of such transmissions and the process of the synchronization procedure are known per se and no longer have to be explained in more detail to the person skilled in the art here.
It is known for protection against premature wear and/or to improve the friction characteristic to provide the friction surfaces of synchronizing rings with a friction layer, said synchronizing rings being produced as a rule from a metal or a metal alloy such as from brass or steel. Very different types of friction layers are in use in this connection, e.g. thermal spray layers made of molybdenum, carbon friction layers or friction layers made from other materials.
Synchronizing units for variable ratio gear transmissions or individual components of synchronizing units are described in a versatile and detailed manner in the prior art.
For example, manual shift transmissions and dual-clutch transmissions are the dominating type of transmissions in the compact and medium-sized vehicle segment today in many applications, in particular with front cross-wheel drives. The shifting characteristic of these types of transmissions is substantially determined by the synchronizing whose principle today as then is based on the so-called Borg-Wamer concept. A high efficiency for realizing shorter shifting times with small shifting forces and a high shifting comfort are demands on the system of synchronizing which are becoming more and more important.
The geometrical configuration of a classical Borg-Wamer synchronizing is determined in this respect by a conflict of objective with respect to efficiency and shifting comfort. A high efficiency is achieved by small conical angles of the friction pairing; the reinforcement effect of the cone generates high synchronization torques despite lower actuation forces. In contrast, the self-locking effect, which prevents a release of the friction surfaces and substantially and noticeably impairs the shifting comfort for the driver, sets a natural limit to the minimization of the conical angle. The optimization of a synchronizing is therefore always a compromise between efficiency and shifting comfort.
The dominant type of synchronizing which is used within the MT transmissions, AMT transmissions and DCT transmissions well known to the person skilled in transmissions is configured in the conventional Borg-Wamer design. Due to the constant demands for higher shifting performance and torques, demanding multiple conical synchronizing devices having two-fold, three-fold and, in specific niche applications, even four-fold friction surfaces have been developed on this basis. These highly developed synchronizing modules have increased performance and torque capacities due to the multiple friction surfaces. Nevertheless, they also increase the complexity, costs and not least, also the weight of the corresponding transmissions.
The EP 0 280 136 A1 discloses a synchronizer element for a stepped-ratio motor vehicle gearing with a base body and a friction body. The friction body is fixed in a positively locking manner to the base body in an axial direction and is supported in a floating manner on the base body in a circumferential direction.
In the non-prepublished EP 2 677 187 A1 a friction ring for a synchronizing unit of a variable ratio gear transmission is disclosed, wherein an inner friction surface extends at a predefinable friction angle and an outer installation surface extends at a predefinable installation angle, each extending conically along the friction ring axis, with the friction angle differing from the installation angle. This friction ring can, for example, be made up of a plurality of friction ring segments.
These already known friction rings can, however, be complex and/or expensive in handling under certain circumstances due to the segmentation if they are mounted at and/or in a synchronizing unit. It can moreover occur that the friction ring and the synchronizer ring cannot be positioned sufficiently exactly with respect to one another in the axial direction or that a displacement in the axial direction takes place in operation.